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Junseo Yoo 2024-06-10 10:47:19 -07:00
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14
docs/index-type-operator.md
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@ -7,7 +7,7 @@ This RFC proposes the addition of one type operator, `index`, which can be used
## Motivation ## Motivation
The primary motivation of this proposal is to allow Luau to define a type by accessing a property of another existing type. For instance, consider the following example code: The primary motivation of this proposal is to allow Luau to define a type by accessing a property of another existing type. For instance, consider the following example code:
```lua ```luau
type Person = { type Person = {
age: number, age: number,
name: string, name: string,
@ -34,7 +34,7 @@ The expected outcome of the index type operator is that it will enhance develope
## Design ## Design
The solution to this problem is a type operator, `index`, that can compute the type based on the static properties of `Person`. Formally, the `index` type operator will take in two arguments: the type to index (indexee) and the type to index it with (indexer). This would allow us to instead write the following code: The solution to this problem is a type operator, `index`, that can compute the type based on the static properties of `Person`. Formally, the `index` type operator will take in two arguments: the type to index (indexee) and the type to index it with (indexer). This would allow us to instead write the following code:
```lua ```luau
type Person = { type Person = {
age: number, age: number,
name: string, name: string,
@ -53,18 +53,18 @@ type idxType2 = index<Person, "age" | "name"> -- idxType2 = number | string
Now, the type of `doSmt()`'s parameter can be defined without declaring a variable `bob`. Additionally, regardless of how the type `Person` grows, `idxType` will always be defined as the union of all the properties. Now, the type of `doSmt()`'s parameter can be defined without declaring a variable `bob`. Additionally, regardless of how the type `Person` grows, `idxType` will always be defined as the union of all the properties.
Error messages will be displayed for incorrect type arguments. If the indexer is not a property in the indexee, Error messages will be displayed for incorrect type arguments. If the indexer is not a property in the indexee,
```lua ```luau
type age = index<Person, "ager"> -- Error message: Property '"ager"' does not exist on type 'Person' type age = index<Person, "ager"> -- Error message: Property '"ager"' does not exist on type 'Person'
``` ```
If the indexer is not a type, If the indexer is not a type,
```lua ```luau
local key = "age" local key = "age"
type age = index<Person, key> -- Error message: Second argument to index<Person,_> is not a valid index type; Unknown type 'key' type age = index<Person, key> -- Error message: Second argument to index<Person,_> is not a valid index type; Unknown type 'key'
``` ```
Note: these errors will be part of the general type family reduction errors since `index` will be built into the type family system. Note: these errors will be part of the general type family reduction errors since `index` will be built into the type family system.
The indexee may be a union type. In this case, the type operator will distribute the arguments to multiple type families: The indexee may be a union type. In this case, the type operator will distribute the arguments to multiple type families:
```lua ```luau
type Person2 = { type Person2 = {
age: string age: string
} }
@ -77,7 +77,7 @@ type idxType4 = index<Person | Person2, "alive" | "age"> -- Error message: Prope
``` ```
In the circumstance that the indexee is a type class or table with an `__index` metamethod, the `__index` metamethod will *only* be invoked if the indexer is not found within the current scope: In the circumstance that the indexee is a type class or table with an `__index` metamethod, the `__index` metamethod will *only* be invoked if the indexer is not found within the current scope:
```lua ```luau
local exampleClass = { Foo = "eight" } local exampleClass = { Foo = "eight" }
local exampleClass2 = setmetatable({ Foo = 8 }, { __index = exampleClass }) local exampleClass2 = setmetatable({ Foo = 8 }, { __index = exampleClass })
local exampleClass3 = setmetatable({ Bar = "nine" }, { __index = exampleClass }) local exampleClass3 = setmetatable({ Bar = "nine" }, { __index = exampleClass })
@ -98,7 +98,7 @@ A drawback to this feature is the possible increase in the cost of maintenance.
## Alternatives ## Alternatives
An alternative design can be depicted from the example below: An alternative design can be depicted from the example below:
```lua ```luau
type Person3 = { type Person3 = {
age: number, age: number,
name: string, name: string,